1. Field of the Invention
This invention relates generally to gas generating compositions and, in particular, to such compositions which are suitable for inflating an air cushion in vehicle restraint systems of the type which utilize such a cushion to protect vehicle occupants upon sudden stopping or deceleration of the vehicle in which they are riding. The air cushion, which is commonly formed of an inflatable bag, acts to prevent the secondary collision between the occupants and interior structural components of the vehicle resulting from the primary collision between the vehicle and another object.
2. Description of the Prior Art
One method commonly proposed for inflating the air cushion of vehicle restraint systems involves the use of a sealed cylinder of compressed gas which communicates with the collapsed bag mounted in the interior of the vehicle. The compressed gas is released by impact responsive actuators or sensors which sense a rapid change in velocity of the vehicle as, for example, when the vehicle collides with another object.
Another method proposed for inflating the air cushion involves the use of an ignitable propellant system, where the inflating gas is generated by the exothermic reaction of the reactants forming the propellant composition.
The bags used in restraint systems of the type described must in order to accomplish their purpose be inflated to a sufficient degree within a very short time span, generally on the order of a few milliseconds. For example, under certain representative conditions only about 60 milliseconds elapse between occurrence of the primary and secondary collisions. In addition, the gas itself must meet several rather stringent requirements. The gas must be nontoxic and non-noxious, and its temperature as it is generated must be low enough not to burn the bag or of itself undermine the mechanical strength of the bag. Furthermore, the temperature of the gas must be sufficiently cool so as not to burn the passengers in the vehicle in the event the bag ruptures.
In systems which utilize an ignitable propellant, the stability and reliability of the propellant composition over the life of the vehicle are highly important. Generally, the propellant composition must possess sufficient stability to temperature, humidity and shock so that it is virtually incapable of being set off except upon deliberate initiation by the actuating sensors employed.
One gas which possesses the required characteristics is nitrogen. In the past, compositions containing sodium azide as one of the reactants have been used to generate pure nitrogen. However, with heretofore known compositions of this nature, the decomposition of the azide leaves a residue of sodium metal which disadvantageously burns spontaneously upon contact with air presenting an extremely undesirable fire hazard to vehicle occupants. Also, the sodium residue disadvantageously exhibits a propensity to react with moisture to produce hydrogen. Hydrogen quite obviously is a highly undesirable by-product, also because of its high flammability characteristics. To eliminate the formation of hydrogen, various organic compounds have been used in previous azide-containing compositions to react with sodium metal residue. But with all such organic compounds, some carbon monoxide is generated as a side product of the reaction. This also is undesirable for the applications herein discussed because of the toxic properties of carbon monoxide.
The present invention provides a novel gas generating composition which, when ignited, generates nitrogen gas without any of the deleterious side-products associated with the reaction of known compositions.